Pressurized oil or transmission fluid performs various functions within an automatically shiftable vehicle transmission. The fluid may act as a coolant to regulate transmission temperature, as a lubricant to reduce frictional wear of dynamic components, and/or as a medium for transmitting mechanical force, such as in the example of a hydraulically-activated clutch pack. Transmission fluid is primarily stored in a main reservoir or sump located within a gear-side portion of a transmission case. From the sump, liquid is drawn through an inlet tube for fluid communication to a hydraulic pump, the pump then pressurizing and circulating the transmission fluid to various areas and components within the vehicle transmission. Two such transmission components are the transmission valve body assembly and the thermal valve assembly.
A transmission valve body assembly contains an array of internal fluid passages or channels for directing pressurized transmission fluid to a variety of components operatively connected within the transmission case, such as valves. In particular, thermal valves are commonly used to regulate fluid levels that may rise due to thermal expansion, and are often positioned on the valve body-side of a transmission case beneath the valve body assembly. The thermal valve assembly is disposed within an opening, orifice, or passage leading from an auxiliary sump volume of the valve body-side of the case and the main sump volume located on the gear-side portion of the case. The thermal valve assembly actuates or cycles when heated, thereby closing and sealing off the fluid passage connecting the auxiliary sump portion to the main sump. Because this fluid passage is closed, transmission fluid levels gradually rise within the volume of the valve body-side portion until that side of the case is nearly full.
When rising transmission fluid levels reach the top of the valve body-side portion of the transmission case, the fluid will escape or exit through any available port or orifice, such as a fluid exit passage or gate in the case wall typically positioned above the valve body. After the overflow fluid passes to the gear-side portion of the transmission case and makes its way into the main sump, the fluid may incidentally contact various moving or dynamic transmission drive components, such as chains, belts, or planetary gear sets and their constituent gear elements. The combination of fluid motion and contact with the dynamic drive components may produce fluid aeration, or the entrainment of tiny air pockets or bubbles within the fluid.